Process and apparatus for fractionation of a gaseous mixture employing side stream withdrawal, separation and recycle

ABSTRACT

In fractionating a gaseous mixture, e.g. an acid gas and hydrocarbon, in a column wherein, during distillation, components of the gaseous mixture tend to form a substantially azeotropic mixture, or at least one of the components of the gaseous mixture tends to freeze out, withdrawing a side stream fluid from the column during fractionation, and separating said side stream fluid, e.g. by membranes or scrubbing, so as to remove preferentially at least a portion of one of the components of the substantially azeotropic mixture, or of the at least one of the components tending to freeze out, and recycling resultant depleted side stream to said fractionating column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 06/866,293filed May 23, 1986, now abandoned, which is a continuation ofapplication Ser. No. 06/610,725 filed May 16, 1984, now U.S. Pat. No.4,599,069, issued July 8, 1986.

BACKGROUND OF THE INVENTION

This invention relates to a system for the fractionation of a gaseousmixture, especially hydrocarbon mixtures, by distillation, especially atlow temperatures wherein either components of the gaseous mixture form asubstantially azeotropic mixture or at least one of the components ofthe gaseous mixture tends to freeze out.

By way of definition, the term "distillation" used herein is intended toembrace rectification as well as other conventional distillationprocesses. The term "substantially azeotropic mixture" is intended toembrace true azeotropic as well as approximate azeotropic mixtures,e.g., equilibrium mixtures of CO₂ with H₂ S which at certain constantpressures exhibit CO₂ concentrations in the vapor phase that areeverywhere greater than those in the liquid phase, but where theconcentrations of CO₂ in the vapor and liquid phases are nearlyidentical over an extended composition range.

Fractionation processes pertinent to this invention have been describedin "Hydrocarbon Processing", May 1982, pp. 131-136. If a gaseous mixtureis to be fractionated wherein the components during distillation form asubstantially azeotropic mixture, then product purity is limited by theconcentration of the components at the azeotropic point. In thedistillation of other gaseous mixtures at low temperatures, where thereflux liquid is insufficient to maintain all of the components insolution at the cold temperatures, one or more of these components beginto freeze out, thereby terminating the distillation.

One suggested solution to the problem proposed in the aforementionedprior publication resides in admixing to the reflux liquid in the columnan additional component, such as n-butane or a mixture of light or heavyhydrocarbons. Such admixture has the effects of increasing the amount ofreflux liquid, altering the composition of the liquid, and increasingthe temperature in the column. These effects, in the one case, suppressthe deposition of solids which would otherwise freeze out, and in theother case with azeotropic mixtures results in an improvement of thepurity of the fractionation products. The added components are thenwithdrawn from the distillation column and recovered in a supplementaldistillation stage.

This suggested process, however, has several disadvantages. For example,in the separation of the addition component in the downstreamsupplemental distillation stage another azeotropic mixture occurs inpart, for example ethane and carbon dioxide. Consequently, an additionalcomponent must also be introduced into this supplemental downstreamdistillation stage and this increases the cost of the process.

A further disadvantage resides in that the additionally introducedcomponent causes dilution of the reflux liquid in the column, thusincreasing the work of separation in the column. Still anotherdisadvantage is that the additional component that must be fed to thecolumn as a cold liquid is heated, during its downward flow within theto the higher temperature of the liquid in the column p, so that a largeportion of the cold value is dissipated.

SUMMARY

An object of one aspect of this invention is to provide one or moreprocesses of the type mentioned hereinabove but which exhibit improvedeconomics for the separation of gaseous mixtures containing componentstending to freeze out or for substantially azeotropic mixtures.

According to an object of another aspect of this invention, apparatus isprovided for conducting such improved processes.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

Though the process of this invention is applicable to gaseous mixturescontaining substantially azeotropic mixtures or components which tend tofreeze out, it is to be understood that the two types of gaseousmixtures exhibit different problems, and that in the absence of thisinvention, there would be no suggestion that the invention disclosedherein could be used for both types of mixtures.

To attain the objects of this invention, a system is provided whereinduring distillation, at least a partial stream containing components ofthe substantially azeotropic mixtures, and/or of the component(s)tending to freeze out, is removed from the column, subjected toseparating to deplete the removed distillation medium in a component ofsaid substantially azeotropic mixture or in said component(s) tending tofreeze out, and resultant depleted distillation medium is recycled tothe distillation column.

In the process of this invention, a vapor-phase or liquid fluid iswithdrawn from the fractionating column, preferably as a side stream,e.g., from a plate above the bottom plate and below the top plate, thecomposition of the fluid is changed, and then the resultant alteredfluid returned into the column in the vapor of liquid phase. In directcontrast to the previously known process, altering of the compositiontakes place by separating specific components from the fluid to bereturned to the column, rather than by adding an additional component.

It is possible by the process of this invention to fractionate gaseousmixtures tending to form azeotropic mixtures or tending to produce soliddeposits in the fractionating column. This eliminates the difficultiesinherent in the prior art method of introducing an additional component.Distillation in this invention is thus improved by separating part ofthe gaseous components. ponents.

In particular, when a component which tends to freeze out is removed,not only is there an advantage because of the reduction in theconcentration of this component in the column, resulting from theincreased reflux liquid from the recycled stream, but there is also anadvantage because the effective amount of this component is reduced,which reduces the work required for separating this component in thecolumn. This compensates for the work required to separate the componentin the supplementary separation process.

In an advantageous further aspect of the process of this invention, thefluid to be returned is withdrawn at certain levels, e.g., in a columnof 20 theoretical plates, the fluid is withdrawn at anywhere from the2nd to the 19th plate, and especially at the middle in the case ofapproximate azeotropic mixtures, whereas in the case of freezing ofcomponents it is withdrawn from about the l0th to the l9th plates,especially at approximately the middle or upper third of the column.

The recycle of the fluid admittedly entails a certain dilution of theprocess streams in the column, but this dilution is restricted to alimited zone of the column, whereas in the conventional introduction ofan additional component, the entire column, down to the sump, istraversed by a stream of dilute reflux liquid. The cold values containedin the withdrawn fluid can be utilized oor cooling the gaseous mixtureto be fractionated or, in conjunction with a heat pump, for cooling thehead of the column. Therefore, the process as conducted according tothis invention not only prevents freezing out, but also improves theeconomics of the distillation step.

In a preferred embodiment of another aspect of the process of thisinvention, separation of the withdrawn distillation medium is conductedby a physical or chemical scrubbing step.

In another advantageous aspect of the process of this invention, theseparation is conducted by diffusion on semipermeable membranes.

Because of their substantially different diffusion characteristics, acidgases, such as H₂ S or CO₂, can be separated by diffusional processesfrom methane, ethane and/or higher paraffins. The use of semipermeablemembranes for this purpose is highly economical with respect to bothapparatus costs and energy consumption. Such semipermeable membranes areconventional and include, but are not limited to: cellulose acetate,polysulfanone, silicone rubbers, polyesters. Cellulose acetate ispreferred for the separation of CO₂ and C₂ H₆, but the choice ofmembrane will depend on the actual components to be separated, and theoperating conditions of the membrane, and other equipment.

For further details on the separation of gases by membrane permeation,reference is made, for example, to Perry and Chilton, ChemicalEngineer's Handbook, 5th Edition, 1973, McGraw-Hill, pp. 17-34 through17-38.

In a preferred further aspect of the process of this invention, thegaseous mixture to be fractionated contains an acid gas and ahydrocarbon. Suitable acid gases include, but are not limited to, CO₂and H₂ S; suitable gaseous hydrocarbons include, but are not limited to,methane, ethane, and propane. Examples of gaseous mixtures to befractionated in percent by volume are: (A) CH₄ (about 5 to 20%), CO₂(about 50 to 90%) and C₂ H₆ (about 2 to 20%); and (B) C₄ H₁₀ (about 10to 50%), C₃ H₈ (about 10 to 50%), H₂ S (about 10 to 50%) and C₂ H₆(about 10 to 50%). (A) is a mixture containing components which tend tofreeze out, and (B) is a mixture containing substantially azeotropicmixtures.

Advantageously, the gaseous mixture contains an acid gas as well as twoother components having different boiling poits, and in particular, thedistillation medium to be recycled is withdrawn from the column at thezone of the highest concentration of the component having theintermediate boiling point.

If the mixture contains, for example, an acid gas, as well as methaneand ethane as the other components, then the medium to be recirculatedis withdrawn from the column at the zone of the highest ethaneconcentration in the latter. Since ethane is capable of dissolving thelarge amounts of acid gas, and moreover increases the temperatures atthe column plates, this aspect of the invention is of special advantagefor the fractionation. Although a maximum in the ethane concentrationwould occur in the column even without the separation of the componenttending to freeze out, the separation steep of this inventionsubstantially enhances this effect. Thus, it would be necessary todetermine the location in a column of the maximum ethane concentrationand at substantially that location, withdraw the side stream from thedistillation medium.

The formation of a high ethane concentration is especially beneficialif, in a further aspect of the process of this invention, the depleteddistillation medium is returned into the column above the point ofwithdrawal.

A high ethane concentration not only prevents freezing and of acid gas,but also additionally increases the amount of reflux in the column andmoreover increases the efficiency of the diffusion process -- ifsemipermeable membranes are used for separation purposes. The reason forthis increased efficiency is that the relative permeability of, forexample, cellulose acetate membranes for, e.g., CO₂ /CH₄ -mixtures isabout ten times as low as for CO₂ /C₂ H₆ -mixtures. As a consequence,the acid gas can be separated from the withdrawn dsstillation mediumwithout a large loss of ethane. By using the operating procedure of theprocess described with reference to the hereinafter described example ofseparating an acid gas-methane-ethane mixture, the largest part of themethane can also be recovered.

Since ethane has a higher concentration in the liquid then in the vaporphase, the distillation medium to be recycled is suitably withdrawn fromthe column in the liquid phase. Inasmuch as liquid ethane is a goodsolvent for the acid gas component tending to freeze out, theacid-gas-depleted distillation medium is also suitably returned into thecolumn in the liquid phase.

In the fractionation of gaseous mixtures tending to form azeotropicmixtures, it may be advantageous under certain circumstances to returnthe fluid to be recycled at below the point of withdrawal into thecolumn

The exact positions where the fluid should be removed and returned tothe column depend upon the compositional profiles in the column and uponthe influence of compositional changes on the phase equilibria of thefluids within the column, and upon the specifications for the productsof the column and supplementary separating process. The recycling of thedepleted stream to a point below the point of removal would normally berequired in the case of an approximate azeotropic mixture occurring inthe lower part of the column, said mixturing being such that animprovement in separation may be achieved by addition of a componentpresent in the upper part of the column.

Recycling of the depleted stream to a position above the point ofremoval would normally be required in the case of either the freezing ofone or more components or in the case of an approximate azeotropicmixture occurring in the upper part of the column, said mixture beingsuch that an improvement in separation may be obtained by addition of acomponent present in the lower part of the column.

In an advantageous further aspect of the process of this invention, theacid gas component of the distillation medium recycled into the columnis at most 5 and preferably no more than 3 mol% of the recycled medium.The acid gas proportion may, in any case, only be of such a magnitudethat no solids are precipitated during recycling into the column.

If the fluid withdrawn from the column is conducted over semipermeablemembranes, it must be compressed beforehand so as to at least compensatefor the resultant pressure loss. Compression also simultaneouslycontributes to part of the refrigeration required in the process. As aresult of the increased pressure ratio the separation of the componentscan be made sharper. A high ethane concentration also contributes to asharp separation because the rate of permeation of ethane is very lowcompared to the acid gases, e.g., CO₂. Alternatively, for a constantpressure ratio, the pressure on the low pressure side of the membranemay be increased. A superatmospheric pressure is desirable to preventingress of air which could produce explosive mixtures with anyhydrocarbons contained in the permeate stream; it also obviates the needfor an expensive and energy intensive vacuum pump.

In a further embodiment of the process of this invention, the gaseousstream diffusing through the semipermeable membranes, which streamcontains essentially acid gas, e.g., CO₂, is mixed with the sump productof the column also rich in CO₂. These CO₂ streams are reused, forexample, in tertiary oil recovery. Details of a tertiary oil recoveryprocess employing CO₂ or the like are found in the literature, e.g., B.C. Price, F. L. Gregg "CO₂ /EOR: from source to resource" 62nd AnnualGPA Convention Mar. 14-16 1983, San Francisco, published also in Oil &Gas Journal, Aug. 22, 1983.

In a preferred further development of the process of this invention, thegaseous mixture to be fractionated comprises natural gas or a gaseouspetroleum component, e.g., propane. This invention is especiallyapplicable to the distillation of a gaseous mixture which is a gasrecovered from a tertiary oil recovery process, wherein CO₂ is injectedas the motive gas into a well hole for expelling and reducing theviscosity of oil as discussed above. Thus, (a) the feed gas into thewell hole and (b) the withdrawn gas from the formation respectivelycomprises: (a) the separated CO₂ fraction and (b) the feed gasassociated with the distillation process of this invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1, 2 and 5 are schematic flowsheets showing three differentpreferred embodiments of the process of this invention and the apparatusassociated therewith;

FIG. 3 is a solubility diagram of CO₂ in liquid in various zones of adistillation column; and

FIG. 4 is a C₂ H₆ concentration diagram.

DETAILED DESCRIPTION

In the process illustrated in FIG. 1, there is introduced into column 2there is introduced via conduit 1 a gaseous mixture having the followingcharacteristics:

composition (by volume): 50% CO₂, 45% CH₄, 3% C₂ H₆ , 2% C₃hydrocarbons;

pressure: about 36 bar;

temperature: about 300 K;

throughout rate: 100 mol/sec.

After being cooled to about 230 K in a heat exchanger 3, the gaseousmixture is introduced into column 2, operated under a pressure of about35 bar, and is fractionated therein into a methane-rich, gaseousfraction 4, removed at the head of column 2, as well as into a liquidfraction 6 rich in carbon dioxide, withdrawn at the bottom of column 2.Fractions 4 and 6 are heated in heat exchangers 5 and 7 respectively toabout 300 K.

The methane-rich fraction 4 contains at most 2% CO₂ ; the withdrawnquantity is about 43 mol/sec. The CO₂ -rich fraction 6 contains about87% CO₂, the withdrawn quantity is about 43 mol/sec. The pressure of thetwo fractions 4 and 6 is respectively about 34 bar.

The temperatures in column 2 range between about 180 K (head) and about260 K (bottom). In the zone containing the head condenser 8,(approximately in the upper third of column 2) a portion of the CO₂would ordinarily be precipitated in solid form owing to the conditionspresent at that location.

In order to avoid such precipitation, a liquid stream 9 is withdrawnfrom column 2 approximately at the level of the middle of the column,the characteristics of this stream being 42% CO₂, 21% C₂ H₆, 37% CH₄, 25mol/sec. This stream 9 is divided into two partial streams which areheated and vaporized in heat exchanger 10 and/or heat exchanger 11 andare subsequently recombined. The stream 9 is then compressed in acompressor 12 to about 50 bar and, after removal of the heat ofcompression in a cooler 13, introduced at about 300 K into a membraneseparation unit 14 provided with semipermeable membranes. Based on thedifferential permeabilities of the membranes for the various gaseouscomponents, a fraction 14 with about 73% CO₂ (14 mol/sec with about 1.5bar) is removed on the low-pressure side of the separator 14, whereas afraction 16 depleted in CO₂ (about 3% CO₂, 45% C₂ H₆, 52% CH₄, 11mol/sec) is obtained on the high pressure side of the separator Thesemipermeable membrane is e.g., cellulose acetate.

After the predominant part of the CO₂ has thus been separated, theremaining gas is cooled in heat exchanger 10, expanded, and thereby atleast partially liquefied, and finally returned into column 2 below(conduit 17) or about (dashed-line conduit 18) of the head condenser 8.If desirable, the stream 16 can also be recycled in an entirely gaseouscondition to the column. This could be advantageous with respect to theconstruction of the heat exchangers 10, 33 or 8 inasmuch as it might beadvantageous to condense the recycled stream in the condenser of thecolumn rather than in an external exchanger. This depends on a number offactors such as heat loads, enthalpy-temperature diagrams, gas-liquidvelocities, construction limits, etc. It might also be advantageous withrespect to distribution of the recycled C₂ H₆ within the column orcooling coils.

Furthermore, if desired, the CO₂ -rich fraction 15, after compression,can be mixed with the CO₂ -rich fraction 6 from column 2. In this way, alarger quantity of CO₂ can be passed, for example, to a tertiary oilrecovery system and the gas recovered therefrom can after dehydratio andother necessary preprocessing be treated as feed gas in conduit 1.

In the same apparatus operating in analogous manner, it is also possibleto rectify a gaseous mixture which, during fractionation in column 2,would otherwise form a substantially azeotropic mixture. Examples ofsuch a mixture include but are not limited to mixtures of CO₂ with C₂ H₆and other hydrocarbons, and H₂ S with C₂ H₆ and other hydrocarbons.

FIG. 2 illustrates a system similar to that of FIG. 1, but with thedifference being that separation of CO₂ from the liquid stream 9withdrawn from column 2 takes place by scrubbing rather than by membraneseparation. (Corresponding structural components bear reference numeralsidentical to those in FIG. 1.)

The liquid stream 9 taken from column 2 is heated and vaporized in heatexchanger 20 and subsequently compressed in compressor 12. As analternative to compressor 12, it is also possible to provide a liquidpump upstream of heat exchanger 20.

In order to separate the CO₂, the compressed gas is fed to a scrubbingcolumn 21 where the CO₂ is absorbed by a chemical or physical scrubbingmedium. Examples of a physical scrubbing agent include, but are notlimited to, N-methyl-2-pyrrolidone, and for a chemical scrubbing medium,methyldiethanolamine. The preferred scrubbing medium ismethyldiethanolamine in water.

A gas extensively free of CO₂ is removed in conduit 32 from the heat ofthe scrubbing column 21; after being cooled in cooler 33, this gas isexpanded and reintroduced into column 2 either via conduit 34 below thehead condenser 8 or via the conduit 35, illustrated in dashed lines,above the head condenser 8. Recycling takes place, depending uponrequirements, in the gaseous or liquid condition. Depending on thescrubbing medium used and column temperatures it may be necessary toremove traces of the scrubbing medium from the stream 32 beforereturning it to the column in order to prevent precipitation of solidphases in the heat exchanger 33, valves 34 or 35 or column 2. In thecase of methyldiethanolamine dissolved in water, a solid absorbent maybe used.

At the bottom of the scrubbing column 21, a liquid 22 is removedcontaining essentially CO₂ and the scrubbing medium. The liquid isheated in a heat exchanger 23, expanded, and fed to a phase separator 24where a CO₂ -rich fraction 25 is withdrawn overhead. The liquid fraction26 from phase separator 24 is expanded and introduced into a column 27wherein extensive fractionation takes place into a gaseous overheadfraction rich in CO₂ withdrawn via conduit 28 and into a liquid fractionessentially containing regenerated scrubbing medium and withdrawn viaconduit 29. The scrubbing medium in 29 is returned into scrubbing column21 by means of a pump 30, but prior to recycling, cooling of thescrubbing medium is effected in a heat exchanger 31.

FIG. 3 is a diagram of the solubility of CO₂ in the interior of column2, the ordinate being the temperature plotted in Kelvin and the abscissabeing the CO₂ proportion in the liquid, in percent. The curve 36describes the solubility limit of CO₂, below which CO₂ precipitates inthe solid phase. Curves 37 and 38 indicate the CO₂ concentration in theliquid on the column plates, namely for the zone between the head of thecolumn (A) and the point of introduction of the feed stream 1 (B). Thedashed-line curve 37 shows the course of distillation without removal,and curve 38 demonstrates the course with the removal of liquid at 9,and recycling of said liquid after separation of a large part of theCO₂. It can be seen from this illustration that, without utilizing thepresent invention, CO₂ would be deposited in solid form between points Cand D, by operating according to this invention, the CO₂ remains clearlyabove the solubility limit thereby avoiding the deposition of solids.

FIG. 4 is a graph of the concentration of C₂ H₆ in column 2 having atotal of 12 theoretical plates. The number, starting from the bottom ofthe column, of the theoretical plates is plotted as the ordinate againstthe C₂ H₆ concentration of the liquid in molar percent as the abscissa.The arrow 39 symbolically represents the withdrawal of liquid viaconduit 9 on the 9th theoretical plate and return thereof via conduit 17or 34 on the l2th theoretical plate, respectively. Without suchrecirculation, the curve 40 in dashed lines represents the C₂ H₆concentration, whereas solid curve 41 is obtained using therecirculation mode of this invention. It can be seen that the liquid tobe recycled is withdrawn at the point of highest ethane concentration,and that the ethane concentration in the region of maximum concentrationclearly increases by utilizing the process of this invention. Since CO₂is highly soluble in liquid ethane, the increased ethane concentrationis a further factor in preventing CO₂ precipitation.

FIG. 5 shows an example for the separation of a mixture containing anapproximate azeotropic mixture. A mixture 42 of e.g. 40% C₂ H₆, 40% H₂ Sand 20% n C₄ H_(l0) is cooled in a cooler 43 and fed to a distillationcolumn 44. In the column 44, H₂ S and C₂ H₆ tend to form an azeotropicmixture, if the concentration of C₂ H₆ is near 100%.

A liquid side stream 45 is withdrawn from below the middle of column 44,is pumped by a pump 46 through an evaporator 47 into a membraneseparation unit 48. A fraction 54 enriched in H₂ S is removed on thelow-pressure side of the separator 48, whereas a fraction 49 depleted inH₂ S is obtained on the high pressure side of the membrane. Thisfraction containing a high concentration in C₄ H_(l0) is cooled andliquified in a cooler 50, expanded in valve 51, and recycled into column44 above the point of withdrawal. By this, the concentration of C₄H_(l0) in tee column will be increased with the result that the C₂ H₆/H₂ S - azeotrope in the upper part of the column can be broken. C₂ H₆is withdrawn via line 52 from the head of column 44, a liquid mixture ofn C₄ H_(l0) with H₂ S is withdrawn from the sump of column 44 by line53.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages anconditions.

I claim:
 1. A process for fractionating a gaseous mixture bydistillation in a fractionating column wherein, during distillation, atleast one of the components of the gaseous mixture is present in thefractionating column in a sufficient concentration to freeze out in anon-hydrated form as a solid under distillation conditions in the columnin the absence of a preventive step and another one of the components ispresent in the fractionating column in liquid phase, said another one ofthe components having the highest dissolving capacity of the freezablecomponent, said process comprising withdrawing a sidestream fluid formthe column during the fractionation at a point in the column where: (a)the freezable component is at a concentration lower than the maximumconcentration and (b) where the high dissolving component is atsubstantially the maximum concentration, and separating said fluid so asto remove preferentially the at least one of the components andrecycling resulted depleted stream to said frationating column, saiddepleted stream having a higher dissolving capacity for the at least oneof the components than the stream withdrawn.
 2. A process according toclaim 1, wherein said sidestream fluid is withdrawn, height-wise, fromapproximately the middle or upper third of the column.
 3. A processaccording to claim 1, wherein said separating of said sidestream isconducted by a physical or chemical scrubbing step.
 4. A processaccording to claim 1, wherein said separating of said side fluid isconducted by diffusion on semipermeable membranes.
 5. A processaccording to claim 1, wherein the gaseous mixture to be fractionatedcontains an acid gas and a hydrocaroon.
 6. A process according to claim5, wherein the acid gas proportion of the depleted sidestream fluid isat most 5 mol %.
 7. A process according to claim 1, wherein a streamenriched in CO₂ is recovered as bottoms from said fractionating column,another stream enriched in CO₂ is removed from said sidestream fluid,and said two enriched streams are combined.
 8. A process according toclaim 7, wherein said separating of said fluid is conducted by diffusionon semipermeable membranes.
 9. A process according to claim 7, whereinthe gaseous mixture to be fractionated is a gas recycled from tertiaryoil recovery.
 10. A process according to claim 1, wherein the gaseousmixture to be fractionated is a CO₂ -containing motive gas recycled fromtertiary oil recovery wherein a gas enriched in CO₂ is recovered fromthe bottom of said fractionating column and further comprising employingsaid CO₂ -enriched gas as motive gas in said tertiary oil recovery. 11.A process according to claim 10, wherein said separating of said fluidis conducted by diffusion on semipermeable membranes.
 12. A processaccording to claim 1, wherein said gaseous mixture to be fractionatedconsists essentially of methane, carbon dioxide and ethane, the latterbeing the component having the highest dissolving capacity for carbondioxide.
 13. A process according to claim 12, wherein the fluidwithdrawn from the column during fractionation is separated to removecarbon dioxide and to obtain an ethane-enriched liquid, said liquidbeing recycled to the fractionating column.
 14. A process according toclaim 13, wherein the ethane-enriched fluid is recycled into the columnabove the point of withdrawal.
 15. A process according to claim 14,wherein the withdrawn fluid is separated by semi-permeable membranes toremove both carbon dioxide and methane selectively from the fluid,thereby obtaining an enriched stream of ethane which is recycled intothe column.
 16. A process according to claim 14, wherein the gaseousmixture to be fractionated consists essentially by volume of about 5-20%methane, about 50-90% carbon dioxide and about 2-10% ethane.
 17. Aprocess according to claim 1, wherein the component which is removedfrom the sidestream is also the main component which is gathered isgathered in the sump of the frationating column.
 18. A process accordingto claim 1, wherein the gaseous mixture contains CO₂, said CO₂ being thecomponent which is removed from the sidestream.